In the control and command systems of moving objects, of vehicles such as trains, it is necessary to have a reference marker with two objectives, to accurately and safely compute the control curves and to position the moving object. Indeed, if the train follows rails and therefore consequently has only a single degree of freedom, it is nevertheless necessary to give the driver the means of being accurately located during his or her travel along the rail line.
Currently, this location process uses a point information transmission technology comprising two components, namely a beacon on the ground, which is fixed (Eurobalise) and placed along the railway and an antenna embedded on board the moving object. The beacon on the ground is passive and contains, in a memory, its location references. Embedded on the train there is the antenna which has two functions, on the one hand the emission of a radiofrequency identification signal (or RFID for “Radio Frequency IDentification”), the aim of which is to transmit energy to the beacon when the moving object passes by the latter and, on the other hand, reception of the message emitted by the beacon with the energy emitted by the moving object. The passage of a train past one of these beacon therefore triggers the emission of a radiofrequency identification signal, which is detected and time stamped by a positioning system, embedded on board the train, and used to find out, in a point by point manner, the precise position of the train and thus readjust the on board location means, in particular the odometer embedded on board the train.
The drawback of this positioning system is that it constitutes a significant infrastructure burden. Indeed, this system requires the placement of RFID beacons approximately every two kilometres and, once installed, these beacons need to be serviced, which represents a high maintenance cost.
Also known is a project for improving this positioning system using, among other things, a satellite location device (or GNSS, for “Global Navigation Satellite System”) embedded on board the vehicle and supplying a permanent and ongoing location (only in terms of position) as well as virtual beacons located at determined positions along the trajectory of the vehicle. The principle simply consists in tracking the position supplied by the GNSS receiver and in detecting the moment the vehicle arrives at the closest to the positions defined a priori which constitute said virtual beacons.
In a use linked to the rail domain, the location device is configured to trigger a “position pulse” equivalent to that of a Eurobalise, when the position supplied by the location device passes closest to the virtual beacon. This makes it possible to keep compatibility with the detection interfaces of the physical beacons defined by the standards of the European Train Control System (or ETCS).
One drawback of this system is that the radius of protection of the integrity of a GNSS positioning varies between 10 m and 50 m depending on the use or non-use of space augmentation systems such as differential GPS (or DGPS, for “Differential Global Positioning System”) or SBAS (Satellite-Based Augmentation Systems), and according to the augmentation system (DGPS, SBAS, etc.) considered. With such a protection radius, it is difficult to respect the integrity performance objectives imposed by the standards which are less than 5 m and even lower, like the location capabilities of the systems based on the RFID beacons. Furthermore, the quality of the measurements can be degraded by the effect of propagation disturbances or local interferences in the reception environment.
Another drawback stems from the fact that the availability of the satellite positioning signals may be insufficient in the case of vehicles moving on the ground, for reasons of masking or of unavailability of the satellite signals. Thus, the accuracy and the integrity of the location measurements can be significantly worse than those supplied by a robust physical beacon.